Well, I'm not gonna go into details but I can tell you this. Balanced re600 plus hm901 is a match made in heaven - everything sounds sublime with this pair. But with other gear while re600 still sounds good, it's just not the same.
R2A needs more power but sounds great with almost everything (same with R2pro - I think, because I just got them yesterday
).
R2A with dx90 or Oppo HA2 sounds just as good or better than re600/hm901 combo. It's smoother, somehow more cristal clear (it's like all the distortion have been eliminated even though I previously didn't hear it or maybe more precisely didn't pay attention/didn't know it was there in the first place). Bass is superior in both quality and quantity - though quantity is not that different. Even mids seems better. Soundstage while not as wide on R2A as on re600/hm901 in balanced mode, is more holographic/3D.
And then there's background noise. R2A are the only dead silence IEMs I've tried with HA2 (no hiss whatsoever) - of course it's not an issue with dx90 which seems to be dead silence with almost all/if not all.
R2pro... For now let me just say they didn't make as good first impression as R2A but after switching to R2A, R2A just sounded worse
(a bit). I'm guessing it's because pro are more analytical and A's are more musical while sharing the same great qualities.